The present invention relates to apparatus for heating a blast furnace stove having a combustion region and a combustion gas outlet associated with the combustion region.
Blast furnaces are primarily but not exclusively used for the reduction of iron oxide ore to molten iron. The purpose of blast furnace stoves is to provide the blast furnace with a consistent hot-blast temperature, at a desired flow rate, in a safe and environmentally responsible manner over a protracted period of many years. The operation of a blast furnace stove is in principle simple. An air-fuel burner is typically used to burn a fuel gas (typically, predominantly blast furnace gas) and the combustion products are passed through a large mass of refractory brick that captures the sensible heat of the combustion product. Once the refractory bricks have reached the desired operating temperature the burner is shut down and cold air is passed through stove, passing over the bricks, so as to be preheated before being sent to the blast furnace as the hot-blast air. Typically, the stoves are operated in banks of three or four so that some stoves are being heated while others are providing hot blast to the blast furnace.
The blast furnace stoves may have any of a number of different configurations. Typically, each stove comprises a first vertical chamber in which combustion takes place along side a second vertical chamber in which the refractory bricks are located. Such a stove is often referred to as being of the external combustion chamber kind. Stoves in which the combustion region is housed in the refractory chamber are also known. These are called “internal” combustion stoves. In another configuration, the combustion chamber is placed on top of the refractory chamber, typically being located within a dome-shaped structure.
In current practice, there are three main approaches to try to maximise the amount of heat that can be transferred from the stove to the hot-blast. Providing a hot-blast with as high a heat content as possible reduces the coke rate for iron-making in the blast furnace. To achieve a high hot-blast temperature, the refractory or checker bricks in the stoves need to be heated to as high a temperature as is possible within the physical constraints set by the permissible dome temperature of the stove. In consequence, the calorific value of the fuel gas delivered to the burner must be capable of generating a suitably hot flame.
Blast furnace top-gas (often referred to as blast furnace gas) is conventionally the primary fuel used to heat the blast furnace stoves, but the use of this fuel has the drawback that its calorific value is variable, being strongly dependent upon the blast furnace operating practices. The variability of the blast furnace gas' calorific value is such that it is well known to blend the blast furnace gas with a higher calorific value fuel gas such as coke oven gas, converter gas or natural gas, in order to boost its heating value and generate the required flame temperature. It is alternatively known to preheat the fuel gas and air upstream of combustion by the stove burner. Indeed, the combustion gas exiting the stoves during the heating cycle typically has a temperature between 250° and 400° C. and contains about 18% of the energy input to the stoves. In some plants, this relatively hot flue gas is routed to a waste heat recovery unit where a portion of its sensible heat content is capture and used to perform the preheating. Another alternative method of heating the blast furnace stoves is to enrich the combustion air with oxygen. Adding oxygen to replace part of the combustion air increases the flame temperature as, at constant total molecular oxygen flow, the nitrogen ballast in the combustion products is reduced. Commonly, oxygen enrichment of the air is used to facilitate a reduction in the amount of coke oven, converter or natural gas needed to generate the desired flame temperature.
It is desirable to improve the operation of blast furnace stoves but in a flexible manner that is able to take account of changes in the availability and cost of fuel and other gases during an operating campaign.